With the development of dual-mode and multi-mode communicating devices, such as mobile phones, multi-radio support at user side becomes more and more attractive. Many solutions have been proposed to efficiently utilize the multi-radio capability at user side without affecting the quality of upper-level services. For example, handoff to another radio when the serving radio runs into a transportation problem (referred to as “inter-system handoff” hereinafter) has been widely discussed and investigated.
Simultaneously using more than one radios at the user side may provide better QoS support, this is because compared with traditional single-radio case, simultaneous dual/multiple radios can provide larger throughput to the user. In addition, a user can use dual/multiple radios to fulfill his/her requirement which can not be met by only one serving radio because of heavy load. On the other hand, multi-radio diversity can also be used to enhance the “cell-edge” performance when radio(s) at the terminal suffer from bad radio conditions.
Simplified illustrative function block diagram of simultaneous multi-radio devices are shown in FIG. 1. When traffic comes to a generic link layer (GLL) agent at a transmitter node, the traffic will be passed to a switch/multiplexer via a transmitter, and the switch/multiplexer will be responsible for dispatching the traffic into different radios. Specifically, the traffic will go through link layer (LL), Wireless Local Area Network (WLAN), Interface Queue (IFQ), Media Access Control (MAC) 802.11, physical (PHY) WLAN, or go through Radio Link Control (RLC) Universal Mobile Telecommunication System (UMTS), IFQ, MAC UMTS, physical (PHY) UMTS, to the wireless channel. At the receiver side, two flows from different radios will be reordered and combined together in the reordering buffer of the GLL agent and then be passed to GLL receiver to provide upper-level a continuous, robust, and high-speed session.
In FIG. 2, two types of simultaneous multi-radio network architecture with different coupling options are shown.
In FIG. 2a illustrating tight-coupling option, multi-radio resource manager (M-RRM) performs jointly across the RATs (Radio Access Technology). This means some of the RRM functionalities for specific RATs may be integrated into the M-RRM. The generic link layer (GLL) function block is introduced on top of the RAT-specific link layers to facilitate the cooperation and convergence among different radios. In GLL, functions that would allow for the control and configuration of L2 functionalities of different RATs are provided. To converge different link-layers' traffic flow, traffic dispatcher and combiner functions (such as switcher/multiplexer and reordering buffer showed in FIG. 1) are provided to process the traffic flow for upper layers.
In no-coupling option showed in FIG. 2b, two radios are processed and managed separately and can only be converged at an upper layer in which the traffic dispatcher and combiner are provided. Radio resource manager (RRM) in this case is RAT-specific, and GLL only process the traffic and signaling inside the RAT (i.e., GLL is also RAT-specific).
Typically, if one radio is at the cell edge, an intra-radio (intra-system) handover process will be triggered to get connected with a neighboring base station (BS) and resume traffic streams. Hereinafter, an intra-radio handover or an intra-system handover refers to a handover inside one radio, and they can be used exchangeable. An important issue during such intra-system handover is that the traffic break at the user terminal from the time that the user leaves the serving BS to the time that connections are reestablished with a target BS. Such break may cause significant QoS degradation especially for real-time services.
To eliminate the traffic break, soft handover has been introduced into intra-radio handover. In this solution, the user will setup connection with the target BS before it leaves the serving BS, i.e., the handover is performed before its disconnection with the serving BS. Although the traffic break is eliminated in such soft handover solution, it is required that both the serving and target BS transmit the same traffic to/from the handover user device. Hence, the resource consumption is very high. Moreover, soft handover generally can only be performed when the target BS is using the same channel as serving BS (the target BS and the serving BS work at the same frequency). In addition, some present radio systems do not support soft handover at all.
Turning to dual-radio/multi-radio environment, without soft handover, traffic break can also be eliminated by utilizing multi-radio capability at user side. That is, when one radio is in intra-system handover, the traffic on this radio can be switched to another active radio seamlessly. Hereinafter, this solution is called inter-system (inter-radio) handoff. It seems that this solution solves the problem easily.
However, when all the active radios substantially need to perform handover (such as the user moves to a position located at the cell edges of all the radio systems) simultaneously, this solution cannot work. An example is shown in FIG. 3, in which UE (user equipment) 1 and UE2 are dual-radio equipments. In FIG. 3, circles give the coverage of one radio system, and hexagons give the coverage of another radio system. Because UE1 is located at the cell edge in both radio systems, both radios may need to perform intra-system handover. In this case, the above-mentioned solution won't work at UE1 even it has both radio simultaneously working. Therefore, although inter-system handoff can be used to eliminate the traffic break, it cannot work when both/all the radios thereof substantially need to perform handover simultaneously, or when there is a time duration in which both/all the radios thereof are under the execution of handovers.
In multi-radio scenario, there are already many discussions on inter-system handoff (also referred to as inter-system handover or vertical handover), in which a radio will transfer traffic for another radio running into bad conditions. Although such inter-system handoff is a typical process in M-RRM control (for example in RAT selection and congestion control), intra-system handover is seldom discussed as it is generally considered in each RAT-specific RRM. In other words, no one has considered the simultaneous or overlapped intra-radio handover problem in the multi-radio environment.